This invention relates to a railway track ballast tamping device of the type comprising a frame structure adapted to travel on the track rails and at least one tamping unit carried by a support adjustably mounted for vertical movement on said frame structure, said tamping unit consisting of at least one pair of opposite co-operating tamping tools having each one arm and at least one tamping pick secured thereto, an eccentric driven shaft rotatably mounted in said support and rigid with at least one flywheel provided with an out-of-balance weight, and at least one pair of eccentric cranks on which the arms of one pair of tamping tools are pivoted for transmitting a swinging motion to the tamping picks, said arms being coupled to said support through extensible members on the one hand located at a predetermined distance above the upper ends of said arms pivotally coupled to said support and on the other hand pivotally connected to each arm, whereby these arms can rotate about their pivot point on said eccentric shaft.
A tamping device of this character is already known (Austrian Pat. No. 352 169), in which the arms of the tamping tools have a substantially right-angular bent configuration, like a knee lever, and the ends of these arms which are opposite the tamping packers are pivoted on the eccentric cranks of a common eccentric shaft. The extensible coupling members consist of double-acting hydraulic cylinders disposed substantially vertically and pivotally connected at one end to the elbow area of each arm of the tamping tool and at the other end to the tamping-tool support. During the rotation of the eccentric shaft the crank-engaging arm ends accomplish a circular movement and, in contrast thereto, the arm elbows connected to the hydraulic cylinders accomplish an oscillatory reciprocating motion along a circular arc of which the center is at the opposite end of the corresponding hydraulic cylinder which is pivotally connected to the tamping tool support and therefore has a relatively long radius. This motion results from the fact that the elbow of an arm cannot vibrate in a vertical direction since the hydraulic cylinder is held against vertical movement. Thus, a substantially horizontal swinging motion is impressed to the tamping packers or blades, which is particularly advantageous for producing the desired ballast tamping action. Moreover, the construction of this known tamping device, in which the tamping tool is pivoted directly to an eccentric or crank shaft revolving in bearings rigidly fitted in the support, is much simpler and sturdier than other known tamping machines of complicated construction and geometry.
Notwithstanding its relatively simple and sturdy construction, this known tamping device does not meet all the requirements consistent with an optimal exploitation. Thus, a horizontal swinging motion of the tamping packers, in fact of the tampers proper, when the packers have already penetrated into the ballast, yet not necessarily during the initial penetration of the packers into the ballast, is preferable. Furthermore, the mass centers of both bent arms are relatively spaced from each other on either side of the vertical plane of symmetry passing through the axis of the eccentric shaft, whereby the movable masses are so distributed as to substantially eliminate the dynamic forces and torques exerted on the support as a consequence of the eccentric shaft rotation, an awkward arrangement at least when the tamping device is constructed and utilized as a single-tie or double-tie one. An essential requirement laid upon the tamping machine lies in the fact that the support of the tamper tool and therefore also the frame structure of the complete machine or vehicle must not, as far as possible, oscillate as a consequence of vibrating or rotating masses, since this would seriously jeopardize the stability of the machine and the quality of the tampering operation.
Another problem arises with the type of tamping device briefly described hereinabove because care must be taken that the two packers of a same pair of tools oscillate constantly symmetrically, i.e. synchronously with each other. Should these swinging movements become asymmetrical, not only the quality of the ballast tamping work would be seriously impaired but also undesired horizontal vibrations would develop in the complete device.